Chronic treatment with haloperidol modifies the sensitivity of autoreceptors that modulate dopamine release in rat striatum

Effect of chronic haloperidol treatment on synaptic protein mRNAs in the rat brain

Chronic haloperidol treatment caused significant decreases in the levels of synaptotagmin I and IV, synaptobrevin II, syntaxin 1A and Rab 3A mRNAs in the nucleus accumbens but not in the prefrontal cortex medial field, striatum, substantia nigra and ventral tegmental area. No significant changes in SNAP 25 and synaptophysin mRNA levels were observed in any brain region examined. The reduced expression of synaptic proteins may be related to haloperidol-induced depolarization block of mesolimbic dopamine neurons.

Chronic verapamil modifies striatal and frontal cortex dopamine levels

In an attempt to elucidate if a change in dopamine (DA) levels was involved in the antimanic action of verapamil reported in various clinical studies, monoamine concentrations in three brain regions (striatum, frontal cortex and hippocampus) obtained from verapamil-treated rats (10 mg/kg i.p. per day for 21 days) were quantified by HPLC coupled to electrochemical detection, and compared with monoamine concentrations in haloperidol-treated animals (5 mg/kg i.p. per day for 21 days). We have found that verapamil and haloperidol, when injected for 3 weeks to rats sacrificed 2 h after the last injection, decreased the striatal DA concentration to a similar extent. This decrease was not observed in short-term (one injection 2 h before sacrifice) verapamil- or haloperidol-treated rats. Moreover, after such a single injection of verapamil the striatal DA concentration was even increased. The striatal concentration of 3,4-dihydroxyphenylacetic acid (DOPAC) was increased about two-fold by haloperidol, but not by verapamil. This haloperidol-induced increase in striatal DOPAC was similar after one injection and after 21 days of haloperidol administration. Neither verapamil nor haloperidol modified the concentrations of homovanillic acid (HVA) or 3-methoxytyramine (3-MT) in the striatum. In the frontal cortex, chronic verapamil increased the concentrations of DA two-fold, and chronic haloperidol increased the concentration of DOPAC two-fold. The other DA metabolites, namely HVA and 3-MT were not significantly changed. The concentration of serotonin (5-HT) and its main metabolite, 5-hydroxyindoleacetic acid (5-HIAA), in control, verapamil- and haloperidol-treated rats were similar in the three brain regions studied. We conclude that DA autoreceptors are implicated in verapamil's effects on frontal cortex and striatum DA levels; and that the presumed antimanic action exerted by verapamil is due to its long-term effect on these receptors.

Chronic clozapine versus chronic haloperidol treatment: differential effects on electrically evoked dopamine efflux in the rat caudate putamen, but not in the nucleus accumbens

Fast cyclic voltammetry at carbon-fibre micro-electrodes was used to investigate the effects of chronic clozapine or haloperidol administration on electrically evoked dopamine efflux in the nucleus accumbens and caudate putamen of the anaesthetized rat. Stimulation trains were delivered to the median forebrain bundle (60 pulses, 350 microns duration) every 5 min, and the evoked dopamine efflux measured as a function of a) the applied stimulus intensity (range 0.2 mA-1.0 mA), and b) the applied stimulus frequency (range 10 Hz-250 Hz). Chronic administration of either clozapine (20 mg/kg x 21 days, p.o.) or haloperidol (1 mg/kg x 21 days, p.o.) significantly reduced electrically evoked dopamine efflux in the nucleus accumbens over the range of stimulus intensities and frequencies tested. The reduction in evoked dopamine efflux observed in the nucleus accumbens of clozapine- and haloperidol-treated rats showed no statistically significant difference. In contrast, only chronic haloperidol treatment significantly reduced evoked dopamine efflux in the caudate putamen. These findings demonstrate that chronic treatment with either the atypical neuroleptic, clozapine, or the typical neuroleptic, haloperidol, produce long-term changes in mesolimbic dopamine function; actions which may underlie their antipsychotic efficacy. They also provide further evidence that the sparing action of clozapine on nigrostriatal dopamine activity may underlie the lower incidence of extrapyramidal side effects associated with its long-term administration.

The vesicular monoamine transporter is not regulated by dopaminergic drug treatments

The number of neuronal synaptic vesicular monoamine transporters (vesicular monoamine transporter type 2; VMAT2) has been recently proposed as an index of monoamine presynaptic terminal density. The present study investigated the possible regulation of the vesicular monoamine transporter. Rats were treated for 2 weeks with drugs known to influence dopaminergic neurotransmission, including those commonly used in the treatment of Parkinson's disease. Autoradiographic assays were performed using [3H]methoxytetrabenazine, [3H]raclopride, and [3H]WIN 35,428 ([3H]2beta-carbomethoxy-3beta-(4-fluorophenyl)tropane) to measure vesicular monoamine transporter, dopamine D2 receptor and synaptic plasma membrane dopamine re-uptake site bindings, respectively. None of the drug treatments significantly modified levels of vesicular monoamine transporter binding. In contrast, both dopamine D2 receptors and dopamine re-uptake sites were altered by some of the treatment regimens. These data extend preliminary results that suggest the vesicular monoamine transporter is not easily regulated and confirm the plasticity of dopamine D2 receptors and the dopamine re-uptake site. Measures of striatal vesicular monoamine transporter density may, thus, provide objective estimates of monoaminergic innervation in neurodegenerative diseases, unaffected by the use of symptomatic therapies.

Dopamine autoreceptor sensitivity is unchanged in rat nucleus accumbens after chronic haloperidol treatment: an in vivo and in vitro voltammetric study

Fast cyclic voltammetry was used to assess the effects of chronic oral haloperidol treatment (0.7 mg/kg/day for 21 days) on the sensitivity of dopamine autoreceptors in the rat nucleus accumbens both in vivo and in vitro. Evoked dopamine overflow was significantly reduced after chronic haloperidol treatment, but the sensitivity of dopamine overflow to sulpiride, an antagonist at release-inhibiting dopamine autoreceptors, and quinpirole, an agonist at these receptors, was unchanged. The estimated EC50 values for quinpirole and sulpiride (52 and 60 nM respectively) obtained in vitro and the receptor distribution profiles published in the literature suggest that the autoreceptors involved in this modulation are mainly of the D3 subtype. The finding that the reduced dopamine overflow in the nucleus accumbens observed after chronic treatment with a classical neuroleptic is not due to dopamine autoreceptor supersensitivity may therefore be the first functional evidence for unchanged autoreceptor activity in the nucleus accumbens, supporting biochemical findings of a lack of D3 autoreceptor up-regulation after chronic haloperidol treatment. It lends further support to the assumption that the long-term changes occurring during chronic neuroleptic treatment may not lie at the level of presynaptic dopamine receptor regulation.

Stimulus-evoked dopamine overflow in the rat nucleus accumbens is decreased following chronic haloperidol administration: an in vivo voltammetric study

Fast cyclic voltammetry was used to investigate the effects of chronic haloperidol (HAL) treatment on electrically evoked dopamine (DA) overflow in the nucleus accumbens of the anaesthetized rat in vivo. Evoked DA efflux was significantly reduced in rats treated with 1.0 mg/kg per day HAL for 21 days. In rats treated with 0.5 mg/kg per day, evoked DA overflow was reduced, but did not differ significantly from control values. In untreated animals, injection of a single dose of HAL resulted in a significant increase in the DA overflow evoked by subsequent stimulus trains. In contrast, this HAL challenge did not produce a significant enhancement in evoked DA overflow in any of the HAL-treated groups. These results are consistent with the previous reports that basal DA release is reduced after chronic HAL treatment, and show for the first time that chronic HAL administration decreases stimulus-evoked DA overflow in the rat nucleus accumbens in vivo.

Repeated haloperidol increases both calmodulin and a calmodulin-binding protein in rat striatum

Repeated treatment with the antipsychotic drug, haloperidol, leads to an increased behavioral sensitivity to dopamine agonists exhibited upon withdrawal from the drug. An increase in the particulate content of the endogenous Ca(2+)-binding protein, calmodulin, has been demonstrated after repeated treatment of rats with haloperidol. In this study, the anatomical specificity of the effect of repeated haloperidol treatment on the content and subcellular localization of calmodulin was investigated. Responsivity of calmodulin localization to dopaminergic input following drug treatment was assessed by determining the subcellular localization of calmodulin following an in vivo amphetamine challenge before sacrifice. Male, Sprague-Dawley rats were treated with 0.5 mg/kg haloperidol (s.c.) for 3 weeks and withdrawn from the drug for 4 days. Repeated haloperidol increased calmodulin content only in the striatum but altered the subcellular distribution of calmodulin in rat limbic forebrain and frontal cortex. In the latter areas, the soluble calmodulin was increased while the particulate calmodulin was decreased. There was no change in calmodulin in either hippocampus or cerebellum in response to drug treatment. Challenge with the dopamine mimetic, amphetamine, before sacrifice was effective in redistributing calmodulin only in striatum from rats that had been treated repeatedly with haloperidol, demonstrating an increased sensitivity of the translocation process. In order to determine whether a change in a calmodulin-binding protein would accompany the drug-induced increase in calmodulin, striatal calmodulin-binding proteins were examined using a biotinylated calmodulin overlay technique. Repeated haloperidol treatment enhanced calmodulin binding to a 150 kDa protein in striatal membranes. The 150 kDa protein exhibited the same gel mobility and subcellular distribution as myosin light chain kinase immunoreactivity. There was an increase in myosin light chain kinase immunoreactivity in striatal membranes after repeated haloperidol that was apparent in animals withdrawn either 4 or 10 days from haloperidol treatment. Therefore, repeated haloperidol could increase the rat striatal content of calmodulin and potentially that of the calmodulin-binding protein, myosin light chain kinase. Increases in striatal calmodulin and myosin light chain kinase may signal a greatly enhanced sensitivity of actin-myosin interactions after repeated haloperidol that could contribute to haloperidol-induced neurochemical or morphological changes involved in drug-induced synaptic plasticity.

Single preexposure to fluphenazine produces persisting behavioral sensitization accompanied by tolerance to fluphenazine-induced striatal dopamine overflow in rats

Single, previous exposure to a neuroleptic has been shown to produce long-lasting changes in various measures of behavior and neurochemistry upon subsequent drug exposure. The present study examined the effects of a single preexposure to fluphenazine (0.3 or 1.0 mg/kg) or vehicle on the effects of subsequent fluphenazine administration 15 or 30 days later. Intracranial microdialysis was used to assess changes in striatal extracellular dopamine concentrations. Animals were tested for catalepsy response on a horizontal bar test while concurrently collecting dialysis samples. Previous fluphenazine exposure produced a profound tolerance to the effects of subsequent fluphenazine at day 15 or day 30 on increasing extracellular dopamine levels. In addition, animals that had received fluphenazine on the first trial showed significant sensitization to the cataleptic effects of fluphenazine at both time points. Pretreatment with vehicle did not result in tolerance to dopamine overflow and there was only minimal evidence of cataleptic sensitization to a subsequent fluphenazine challenge. Although the tolerance to dopamine overflow may only indirectly relate to behavioral sensitization, these results support the hypothesis that significant behavioral and neurochemical alterations persist for prolonged time periods following single neuroleptic exposure.